Correct Grounding for this system

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Kartracer087

Member
Location
Milwaukee WI
Here is the proposed system, and I want to know if this is the correct way to ground it:

A 480V 3 phase 3 wire bus feeds a 100kva transformer. The secondary is 208Y/120 3ph 4w. The secondary lines are connected to a circuit breaker which is a disconnect for a meter stack. The meter stack provides power to condo units at 208/120 single phase, the disconnects for each unit are in the meter stacks, so the tenant panels are main lug only. If this was the case, then would you run your 2 phase wires, a neutral, and a ground wire back to the meter box and have the tenant panels have isolated grounding busses (not connected to the neutral). So this would mean that the required bonding connection would be made at the meter stack, one bond connection to neutral for each tenant unit. If I'm seeing this right, then the main feeders from the transformer to the meter stack would simply be the 3 phase wires and a neutral. No bonding connection between the neutral and ground would be made at the transformer, only at the meter stack. Do I have the right idea here?
 

iwire

Moderator
Staff member
Location
Massachusetts
All you're bonding and grounding electrode conductors must run to the transformer or the disconnect for the meter stack, beyond that all isolated neutral and grounds.
 

nakulak

Senior Member
if its a poco transformer then op is exactly correct, o/w main bonding jumper could be at tranny or first means (meter disco's), unless the meter discos are poco side and then depends on poco rules ? of course, if this is a commercial project (multiuse or not regular dwellings) and there is a set of plans, then by ivens rules whatever is on the plans would apply whether it was NEC or NASA requirements.
 
All you're bonding and grounding electrode conductors must run to the transformer or the disconnect for the meter stack, beyond that all isolated neutral and grounds.


If the disconnect supplying the meter stack is "immediately adjacent to..." (250.142(B) exception#2)

Then I would say the neutral termination of the meter equipment can also be bonded to the enclosure.
but the grounding and bonding at the disconnect would be treated as a service to any building would be treated.
The MLO panels at each condo would then be treated as a subpanel with the neutral and enclosure/EGC isolated from each other.
 

Kartracer087

Member
Location
Milwaukee WI
I'm studying to be an Electrical Engineer and I was curious to the location of the neutral-ground bond. We are eventually going to have to spec a system in our senior design project where we will have condominium units. I haven't sketched out a formal system yet, but I am more than likely leaning towards having 480V busses feeding substations at different points in the building for the 208/120 distribution to the house panels since this is a typical construction for buildings that aren't crazy tall. I have not dealt with multifamily construction, so I have normally worked with bonding the neutral and grounding electrode conductors at the transformer. Interestingly enough, I've also learned that you are required to have at least one additional neutral to ground bond connection per NEC (250.24 (2)) if your transformer is located outside of the building. Since we are likely going to be primary customers (13.2KV primary with the main transformers located inside of the building), this doesn't apply to us anywhere, but it is noteworthy with respect to the utility transformer comment. It makes me think that NEC is concerned about ground loops from a standpoint of Power Quality since it is OK to have theoretical ground loops by multi grounding the neutral outside of the building. Also most utilities utilize multigrounded systems instead of the single point grounded system that we will be designing for all of our separately derived systems.

Thanks
 
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cpal

Senior Member
Location
MA
I'm studying to be an Electrical Engineer and I was curious to the location of the neutral-ground bond. We are eventually going to have to spec a system in our senior design project where we will have condominium units. I haven't sketched out a formal system yet, but I am more than likely leaning towards having 480V busses feeding substations at different points in the building for the 208/120 distribution to the house panels since this is a typical construction for buildings that aren't crazy tall. I have not dealt with multifamily construction, so I have normally worked with bonding the neutral and grounding electrode conductors at the transformer. Interestingly enough, I've also learned that you are required to have at least one additional neutral to ground bond connection per NEC (250.24 (2)) if your transformer is located outside of the building. Since we are likely going to be primary customers (13.2KV primary with the main transformers located inside of the building), this doesn't apply to us anywhere, but it is noteworthy with respect to the utility transformer comment. It makes me think that NEC is concerned about ground loops from a standpoint of Power Quality since it is OK to have theoretical ground loops by multi grounding the neutral outside of the building. Also most utilities utilize multigrounded systems instead of the single point grounded system that we will be designing for all of our separately derived systems.

Thanks


It might be helpful to break you question into parts of the wiring system. You appear to have a service to the building (check the definition). You also appear to be setting up a number for seperately derived systems. The 480 to 208 step downs are not the same as the utility service that supplie the structure. If you are aware of that I sence from your posts, but I've been wrong before.

See 250. 30, I think it may help.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Well yes i agree with iwire at the service disconnect or at the transformer your choice .

Meaning the grounding electrode conductor from ground rods go to disconnect and you use a bonding jumper to neutral to ground in that disconnect .

Or put a jumper in transformer from neutral to ground your choice go for it .:D
 
"without a splice or joint"

"without a splice or joint"

When I started this question, I was in the Grounding Tread but it went to the wrong thread when posted. Sorry for duplicaton.

From Code section 250.66 and regarding: : "the common grounding electrode conductor remains without a splice or joint"

QUESTION
Does terminating the electrode conductor (with correctly applied T&B compression terminals) from the service entrance neutral to a copper bus bar, from which connections are made in the same manner to approved grounding rods, meet the "without a splice or joint" requirement?

Plan to use two each 2/0 stranded attached to the 400A Service Entrance switch. Then to the 12" x 4" x 1/4" tin plated copper bus bar. Then running 2 each 2/0 stranded to 2 each stainless steel rods 10" x 3/4".

Perhaps overkill but at age 82 I am of the old school. All wire is left over form my more active days in electrical work. 2 each 2/0 = .7296" Dia which is only .0834" smaller than the 500mcm (.8130" Dia) feeder wires (if my calculations are correct).

Will use my T&B TBM6S Shure-Stake crimper with T&B
connectors.

I should not be sitting at the computer so long but read through the forum for a couple of hours tonight. Enjoyable, like being in a library. Thanks so much for all who share their experiences. I have not had a copy of the code for many years... which reminds me, when I started, the code would fit into my breast pocket and I knew almost every word. Very enjoyable, the one whose bottom line goes somewhat like this paraphrased: I am not old, I started in knob and tube. Yep, I actually did start in knob and tube.
"Those were the good old days" but not as exciting as these days with so many good products which I wish that I had had in bygone years.

Thanks and appreciation,

800CM
aka Marshall
WW2 Vet = getting old
Tired but not retired farmer

I use to be steady on my feet and strong
I am still strong
But my deodorant takes care of it
 
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